Sensing apparatus and method for fetching multi-level cell data

ABSTRACT

A reading circuit for a multibit memory cell in a memory array, the memory cell having a threshold gate voltage within a range of one of a first, second, third and fourth predetermined threshold voltages corresponding respectively to one of four states of two bits stored in the memory cell. The reading circuit includes a circuit to provide a gate voltage to the multibit memory cell during a read cycle, the gate voltage having a first level between the second and third predetermined threshold voltages during a first time interval of the read cycle and a second level between the third and fourth predetermined threshold voltages during a second time interval of the read cycle. A sensing circuit is coupled to the multibit memory cell which compares current from the multibit memory cell to a first reference current and to a second reference current, and produces a first output during the first time interval having a first logic state if the current from the cell exceeds the first reference current, and having a second logic state if the current from the cell is less than the first reference current. The sensing circuit produces a second output during the second time interval having a first logic state if the current from the cell is less than the second reference current and greater than the first reference current, and a second logic state if the current from the cell is greater than the second reference current and less than the first reference current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention relates to a multi-stagesemiconductor memory and the sensing of multi-level memory cell data;and in particular to a method and apparatus for sensing multi-level celldata in one read cycle.

2. Description of Related Art

In a conventional memory cell, one bit of data is stored per cell.Popular classes of non-volatile semiconductor memory devices such as ROMand flash memory, have been modified to store more than one bit of datain one cell. This is accomplished by storing more than two thresholdvoltages V_(t) either through different voltage threshold implantationfor devices such as a MROM or by programming in devices such as flashcells.

A draw back to the MLC approach is that there is increased difficulty insensing the various threshold voltages. This compromises the speed ofreading the data. Also, since a more complicated sensing circuit isrequired additional chip area to implement the sense amplifier isrequired, increasing the cost of manufacturing a MLC.

Representative prior art sensing methods are described in U.S. Pat. No.5,721,701 to Ikebe et al. entitled “HIGH READ SPEED MULTIVALUED READONLY MEMORY DEVICE”; and U.S. Pat. No. 5,543,738 to Lee et al. entitled“MULTI-STAGE SENSE AMPLIFIER FOR READ-ONLY MEMORY HAVING CURRENTCOMPARATORS”. These approaches in the prior art for two bit per cellmemory, require three word line voltage levels for sensing the fourpossible combinations of two bits. The three levels are achieved in oneprior art approach by applying the three levels in a three step sequenceto the word line in each read cycle, sensing the cell output for each ofthe three levels. This three step sequence is relatively slow. The threelevels are achieved in another approach by a two step sequence applyinga first fixed word line voltage, and followed by a lower word linevoltage or a higher word line voltage depending on the outcome ofsensing during the first step. The two step technique in the prior artovercomes the slowness of the three step technique, but adds complexitybecause of the logic required to control the word line voltage duringthe second step. Further, the two step sequences of the prior art islimited to a single order of sensing.

What is needed is a novel method and apparatus for fetching MLC datawith a fixed word-line voltage for all bits in the MLC independent ofthe order in which the bits are sensed. What is also needed is a sensingcircuit with reduced complexity and reduced cost.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a novel sensing methodand apparatus to fetch multi-level cell (MLC) data using fixed word linevoltages for both bits, or for all bits in the cell.

The present invention is directed to a novel sensing method andapparatus to sense multiple bits of data from a multi-level cell usingfixed word line voltages for both bits, or for all bits in the cell. Thesensing technique allows sensing a single bit from the cell, does notrequire any particular order of sensing of the multiple bits within asingle cell, and has a sensing margin which is similar to prior artmulti-level cell sense amplifiers.

Thus, the present invention provides a reading circuit for a multi-bitmemory cell in a memory array. In a two bit per cell embodiment, thememory cell has a threshold gate voltage within a range of one of afirst, second, third and fourth predetermined threshold voltages. Thesefour predetermined threshold voltages correspond respectively to thefour states of two-bits stored in the memory cell. A circuit to providea gate voltage to the multi-bit memory cell during a read cycle isincluded. The gate voltage has a first level between the second andthird predetermined threshold voltages during a first time interval ofthe read cycle for sensing one bit, for example the low bit, of themulti-bit data, and a second level between the third and fourthpredetermined threshold voltages during a second time interval of theread cycle for sensing the next bit, that is the high bit in the cell.The grouping of the first and the second threshold voltages and thethird and the fourth threshold voltages are also referred to as the lowthreshold voltage group and the high voltage threshold group,respectively, during the low bit cycle. During the high bit cycle thesecond and third threshold voltages and the first and the fourth voltagethreshold groups are referred to as the inner and outer thresholdvoltage groups, respectively.

A sensing circuit in one embodiment is coupled to the multi-bit memorycell and compares the current from the multi-bit memory cell to a firstreference current and a second reference current. The sensing circuitproduces a first output during the first time interval having a firstlogic state when the current from the first cell exceeds the firstreference current and a second logic state if the current from the cellis less than the first reference. The sensing circuit produces a secondoutput during the second time interval having a first logic state if thecurrent from the cell is less than the second reference current andgreater than the first reference current, and a second logic state ifthe current from the cell is greater than the first reference currentand less than the second reference current. The first and second outputsof the sensing circuit indicate the two bits stored in the multi-bitcell. The sensing method described above can be expanded to include MLCshaving more than four threshold levels. As the number of thresholdlevels increase, additional current sources and logic circuits withcorresponding logic states would be required to determine the datastored in the MLC.

According to one preferred aspect, the sensing circuit includes a firstcomparator connected to receive the first reference current and acurrent from the multi-bit memory cell, and a second comparatorconnected to receive the second reference current and current from themulti-bit memory cell. Logic is coupled to the first and secondcomparators and operates during the first time interval to provide theoutput of the first comparator as the first output, and operates duringa second time interval to provide the output of the first comparator asthe second output if the output of the second comparator has a firstvalue, and to provide the output of the first comparator inverted as thesecond output if the output of the second comparator has a second value.

According to various aspects of the present invention, the first timeinterval for sensing may occur prior to the second time interval, or thefirst time interval may occur after the second time interval dependingon the bit to be sensed, or the preferred implementation of the sensingcircuit.

Also, the present invention provides a method for sensing a particularbit in a plurality of bits in a multi-bit cell that has a threshold gatevoltage within a range of one of a plurality of predetermined thresholdvoltages which includes a highest, a next to highest and a lowestpredetermined threshold voltage. The method involves applying a gatevoltage to the multi-bit cell between the highest and the next tohighest predetermined threshold voltages, and determining a first valuefor the bit if the current from the cell is less than a referencecurrent indicating that the threshold gate voltage is the highestpredetermined threshold voltage, or if the current from the cell isgreater than a reference current indicating that the threshold gatevoltage is the lowest predetermined threshold voltage. The methodincludes determining a second value for the bit if the current from thecell is greater than the first reference current and less than thesecond reference current indicating that the threshold gate voltage isone of the other predetermined threshold voltages in the plurality ofpredetermined threshold voltages.

When the plurality of bits in the multi-bit cell includes two bits,including the particular bit and another bit, the other bit is sensed byapplying a gate voltage to the multi-bit cell between the next tohighest predetermined threshold voltage, and a next lower predeterminedthreshold voltage. In this case, the other bit is determined to have afirst value if the current from the cell is less than a referencecurrent indicating that the threshold gate voltage is higher than thevoltage applied to the cell, and determines that the other bit has asecond value if the current from the cell is greater than the referencecurrent indicating that the threshold gate voltage is lower than thegate voltage applied to the cell. In this way, two bits may be sensedfrom a single memory cell having one of four predetermined thresholdvoltages.

Thus, the present invention provides advantages over conventional MLCsensing by providing a low cost, sense amplification circuit with logicthat is independent of the order of the sensing of the low or high bitdata, with increased reading margin and increased reading speed overconventional MLC sensing circuits.

To achieve these and other advantages of the invention with the purposeof the present invention, as embodied and broadly described, the presentinvention can be characterized according to one aspect as a senseamplifier for a multi-bit memory cell, including logic responsive to abit address of a memory cell to enable one of an inverting and anon-inverting circuit during a reading of the word line determining atleast two bits of data in the memory cell.

A further aspect of the present invention can be characterized as asensing circuit for a multi-bit memory cell in a memory array, thememory cell having memory cells with a plurality of threshold voltages.The sensing circuit determines two bit data of the memory cell bysensing an output of a bit line coupled to the memory cell, the sensingcircuit includes a word line voltage driver responsive to a memory cellbit address and a logic circuit responsive to the memory cell bitaddress for determining the two-bit data. The logic circuit is alsoadapted to respond to a first bit address by providing a first logicstate indicating whether the output of said bit line corresponds to oneof a low threshold voltage group and a high threshold voltage group. Thelogic circuit is further adapted to respond to a second memory cell bitaddress by providing a second logic state indicating whether the outputof the bit line corresponds to one of an outer threshold voltage groupand an inner threshold voltage group.

A further aspect of the present invention can be characterized as asense amplifier for a multi-bit memory cell, including a firstcomparator in communication with said memory cell. A second comparatoris in communication with the memory cell, and a controller is incommunication with the first and second comparator. The controllerincludes resources responsive to the bit address of the memory celldetermining at least two bit data of the memory cell.

A further aspect of the present invention can be characterized as areading circuit for a multibit memory cell in a memory array. The memorycell having a threshold gate voltage within a range of one of first,second, third and fourth predetermined threshold voltages correspondingrespectively to four states of two bits stored in the memory cell,including a circuit to provide a gate voltage to the multibit memorycell during a read cycle. The gate voltage has a first level between thesecond and third predetermined threshold voltages during a first timeinterval of the read cycle and a second level between the third andfourth predetermined threshold voltages during a second time interval ofthe read cycle. The sensing circuit is coupled to the multibit memorycell which compares current from the multibit memory cell to a firstreference current and a second reference current, and produces a firstoutput during the first time interval having a first logic state if thecurrent from the cell exceeds the first reference current. The sensingcircuit also includes a second logic state if the current from the cellis less than the first reference current, and produces a second outputduring the second time interval having a first logic state if thecurrent from the cell is less than the second reference current andgreater than the first reference current. The sensing circuit furtherincludes a second logic state for if the current from the cell isgreater than the first reference current and less than the secondreference current.

A further aspect of the present invention can be characterized as asensing method further including receiving an nth reference current froma nth comparator and receiving current from the multibit memory cell andreceiving a (n+1)th reference current from a (n+1)th comparator and acurrent from the multibit memory cell. The method further provides anoutput based on logic coupled to the nth and (n+1)th comparators,operating during a nth time interval providing an output of the nthcomparator as the nth output, and operating during the (n+1)th timeinterval providing the output of the nth comparator as the (n+1)thoutput if the output of the (n+1)th comparator has a first value, andprovides the output of the nth comparator inverted as the (n+1)th outputif the output of the (n+1)th comparator has a second value, wherein n isa integer equal to 2, 3, 4, . . .

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. The aspectsand other advantages of the invention will be realized and attained bythe structure particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 depicts an operational block diagram of the present invention.

FIG. 2 depicts an operational block diagram of the logic circuit of thesense amplifier of the present invention.

FIG. 3(A) depicts a chip enable pin (CEB) timing diagram during a firstand second cycle of the read operation for the logic circuit of FIG. 1.

FIG. 3(B) depicts a timing diagram for address XM during a first andsecond cycle of the read operation for the logic circuit of FIG. 1.

FIG. 3(C) depicts a timing diagram for non-XM addresses during a firstand second cycle of the read operation for the logic circuit of FIG. 1.

FIG. 3(D) depicts a timing diagram illustrating the word line voltagelevel during a first and second cycle of the read operation for thelogic circuit of FIG. 1.

FIG. 3(E) depicts a timing diagram illustrating the sense amplifieroutput voltage level during a first and second cycle of the readoperation for the logic circuit of FIG. 1.

FIG. 4 depicts a diagram illustrating the logic relationship withrespect to V_(T0), V_(T1), V_(T2) and V_(T3).

FIG. 5(A) depicts a general logic circuit diagram for a memory cell ofthe present invention.

FIG. 5(B) depicts the output of a logic circuit diagram for memory cellcurrent where V_(W)=2.3V and I_(CELL)<I₀ of the present invention.

FIG. 5(C) depicts the output of a logic circuit diagram for memory cellcurrent wherein V_(W)=2.3V and I_(CELL)>I₀ of the present invention.

FIG. 5(D) depicts the output of a logic circuit diagram for memory cellcurrent wherein V_(W)=2.8V and I_(CELL)<I₀ of the present invention.

FIG. 5(E) depicts the output of a logic circuit diagram for memory cellcurrent wherein V_(W)=2.8V and I_(CELL)>I₀ of the present invention.

FIGS. 6(A)-6(F) and FIGS. 7(A)-7(F) illustrate timing diagrams for thevoltages applied to the selected memory cells during sensing of the lowand high bits of a two bit memory cell according to alternativeembodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A detailed description of the present invention is provided with respectto the figures, in which FIG. 1 provides a simplified block diagram of amulti-level memory array according to the present invention.

In FIG. 1, an array 140 of multi-bit memory cells is included. Themulti-bit memory cells may include mask ROM cells which have beenmanufactured with cells having a plurality of threshold voltages, in apreferred embodiment for threshold voltages V_(T0), V_(T1), V_(T2) andV_(T3). In alternative systems, rather than mask ROM other multi-levelcell embodiments may be utilized, for example multi-level programmedfloating gate memory cells may be utilized in a preferred embodiment.

With reference to the block diagram in FIG. 1, the array includes ROM orfloating gate cells, or other multi-level cells. An address is suppliedon line 105 to a decoder 110. The decoder applies row addresses on line115 to a two step word line driver 130. Column addresses are applied online 120 to a column select circuit 145. An address signal XM issupplied by the decoder 110, corresponding to a single bit in theaddress 105 for example, to the two step word line driver 130, and to asense amplifier circuit 165. The two step (or one step in someembodiments) word line driver 130 selects one of a plurality of wordlines 135 which is coupled to the array 140. The column address on line120 causes the column select circuit 155 to select one of a plurality ofdata lines 150 to be selected onto a data line 160. Sense amplifier 165determines the value of the data in the selected memory cell 145 inresponse to the address bit XM selected either the high bit or the lowbit for a two bit cell. The output S(OUT) is provided on line 170 fromthe sense amplifier.

Also included in the diagram is a source voltage supply circuit 141which applies a source voltage to memory cells in the array. At leastone of the word line driver and the source voltage supply are responsiveto the address bits and control logic in the device to apply agate-to-source voltage to the selected memory cells. The gate-to-sourcevoltage used for sensing the value of the multiple bits in the cell isresponsive only to the address bits and the control logic in the device,and does not depend on the output of the sense amplifier from onesensing step to determine the word line levels to be driven.

Thus, an one bit in a multi-level cell may be sensed in a single readcycle with a word line voltage predetermined or fixed, without requiringthe sensing of other bits in the cell. Table 1 set forth belowillustrates the sensing logic in a preferred embodiment of the presentinvention. According to this embodiment, the supply potential VDD isapproximately 3.3 volts. A two bit cell has one of four thresholdvoltages V_(T0) through V_(T3). V_(T3) is approximately 4 volts, V_(T2)is approximately 2.5 volts, V_(T1) is approximately 2 volts, and V_(T0)is approximately 0.8 volts in this embodiment. The low bit correspondingto address XM=0 and the high bit corresponding to address XM=1 aredefined in the right hand two columns of the table. Of course othercombinations of supply potential and threshold voltages may be utilized.Also cells having more than four threshold voltages may be utilizedaccording to the present invention.

TABLE 1 Threshold Voltage VT(VDD = 3.3 V) Low Bit High Bit V_(T3)   4 V0 0 V_(T2) 2.5 V 0 1 V_(T1) 2.0 V 1 1 V_(T0) 0.8 V 1 0

Thus, when the low bit data is being determined investigation of currentI_(CELL) is confined to whether there is an I_(CELL) current or not. Byraising V_(W) to 2.3V which is just above the 2.0V of V_(T1), asdepicted in TABLE 1, either there will be an I_(CELL) current or not.This is because V_(W) will either be below the threshold of V_(T2) andV_(T3), which is below the threshold required to produce an I_(CELL)current. By choosing V_(W) between V_(T1) and V_(T2) determination canbe made that the logic for the low bit corresponds to one of regions 410(lower threshold voltage group) or 420 (high threshold voltage group) ofFIG. 4. Thus, as depicted in TABLE 1, V_(T0) and V_(T1) are assigned alogic state of “1” and V_(T2) and V_(T3) are assigned a logic state of“0” for the low bit address of the cell. It should be noted that thelogic states depicted in TABLE 1 could be reversed, in that each onecould be made a zero and each zero a one. Of coursed the logic circuitwould have to be altered to accommodate such a change, but the sameresult of identifying the data contained in the cell would be achieved.

With respect to the high bit determination, a V_(W) of 2.8V is appliedto the n-bit cell, which is greater than V_(T2) but less than V_(T3). Inthis scenario an I_(CELL) current will flow for any cell level with athreshold voltage below V_(W) of 2.8 v, i.e. V_(T0), V_(T1) and V_(T2).For the regions of the multi-level cell with voltage thresholds ofV_(T1) and V_(T2) only a small to moderate I_(CELL) current will flow.For the region of the multi-level cell with a voltage threshold ofV_(T0) a larger I_(CELL) current will flow. The sense amplifier and itsassociated logic need only be able to differentiate between no currentflow, which is very easily done, and a large and small I_(CELL) currentflow. A zero current flow corresponds to region 440 of FIG. 4 and alarger I_(CELL) current flow corresponds to region 450. Regions 440 and450 (outer threshold voltage group) are assigned logic states of “0”. Nodifferentiation is performed between the V_(T1) and V_(T2), whichcorresponds to region 430 (inner threshold voltage group) of FIG. 4 andare assigned a logic state of “1”.

Based upon the assigned logic states a determination of n-bit cell datacan be performed without regard to the order in which the low bit or thehigh bit data is retrieved. As previously noted the logic state of a “1”or “0” assigned is arbitrary as long as the relationships depicted inFIG. 4 are maintained between regions 410, 420, 430 and 440, the resultwill be exactly the same, assuming the logic circuit is modified toreflect the change of the logic state designations.

FIG. 2 is a simplified block diagram of the sense amplifier. FIGS. 3Athrough 3E illustrate the timing of the control signals for the circuitof FIG. 1. In combination, the present invention can be understood withreference to FIGS. 2 and 3A through 3E. FIGS. 3A through 3E illustratethe chip enable signal (CEB), which is set low at 301 at the beginningof a read cycle. The address bit XM is low during a first interval 302of the read cycle and high during a second time interval 303 of the readcycle. The other address bits are valid during the first time interval302 and the second time interval 303. The two-step word line driver 530produces a word line voltage which goes to a first gate voltage at level304 during the first time interval 302 and to a second gate voltage atlevel 305 during the second time interval 303. The output of the senseamplifier 550 for the low bit is provided during time interval 306 andfor the high bit is provided during time interval 307.

In this example, the cycle for XM=0 and the cycle for XM=1 areillustrated as part of a single read cycle. In various embodiments, theread cycles may be completely independent allowing for sensing of thelow bit, independent of sensing of the high bit and vice versa. Also,the order in which the low bit and high bit are sensed may be reversed.

With reference to FIG. 2, and the timing diagram of FIGS. 3A through 3E,the operation of the sensing circuit may be understood. In FIG. 2,current from the cell I_(CELL) is provided to a first comparator 210 andsecond comparator 220. The first comparator 210 compares the cellcurrent to a first reference current I₁, and the second comparator 220compares the cell current to a second reference current I₀. The outputof the first comparator 210 is supplied to an inverter control circuit230. Also, the address bit XM is supplied to the inverter controlcircuit 230. The sense amplifier includes a first output path includinginverter 240, and a second non-inverting output path. Thus, the logic isable to provide the output of the comparator 220 through inverter 240 asthe output S_(OUT) during one state, and provide the output of thecomparator 220 non-inverted in a second state. The inverter controlcircuit 230 selects the data paths to determine the state of the outputsignal.

When the signal XM is zero, the inverter control circuit always selectsthe path not including inverter 240 so that the output of the comparator220 is provided as the output. When XM=1, the inverter control circuitselects inverter 240 as the output path if the output of the comparator210 has one state, and selects the data path not including inverter 240if the output of the comparator 210 has the other state. In operation,for sensing the low bit the address bit XM is zero, and a word line isstepped to a threshold voltage in the preferred embodiment ofapproximately 2.3 volts. This is a threshold voltage between V_(T2) andV_(T1). In this case, if current from the I_(CELL) is less than I₀, thecomparator 220 will output a logic one. This output will be provided asone on line S_(OUT). Conversely, if the current from the I_(CELL) isgreater than the current I₀, then the comparator 220 will output a zero,which will be provided as the signal S_(OUT). Thus, as shown in FIG. 4,the low bit is zero if the threshold voltage is higher than the voltageapplied during the low bit read cycle, and one if the threshold voltageis less than the voltage applied during the low bit read cycle.

During the high bit read cycle, the comparator 210 is utilized tocontrol the inverter control logic 230. In this case, the word linevoltage is set at a voltage of approximately 2.8 volts in this example,which is a voltage between the V_(T3) and the V_(T2). That is, a voltagebetween the highest threshold voltage of the multi-bit cell and thenext-to-highest threshold voltage of the multi-bit cell. In this case,if the current from the cell is greater than the reference current I₁,then the output of the comparator 210 will cause the inverter controlcircuit to select the data path including the inverter 240. If thecurrent from the cell is less than the reference current I₁, then thecomparator 210 will cause the inverter control circuit 230 to select theoutput of the comparator 220 directly as S_(OUT). If the current fromthe cell is less than the reference current I₀, it must also be lessthan the reference current I₁ and the output of the comparator 220supplies a logic 1 for the output S_(OUT). If the cell current I_(CELL)is greater than the current I₀, then the output of the comparator issupplied as the output S_(OUT) provided the cell current is less thanthe reference current I₁.

The reference current I₁ is utilized only for sensing the high bit, whenXM=1, and the word line is driven to the threshold voltage between thehighest and next-to-highest levels of the cell to be sensed. Thus, thereference current I₁ is set to a level which is the minimum currentexpected from a memory cell having the threshold voltage V_(T0) when theword line voltage is set to 2.8 volts in this example. If the currentfrom the cell exceeds this minimum, then it can be assumed that the cellcurrent is generated by a memory cell having the threshold voltageV_(T0).

The reference current I₀ is utilized during both the low bit and highbit sensing. Thus, it is set at a level which is less than the minimumcurrent to be sensed from a memory cell having a threshold of V_(T1) orV_(T0) when the word line voltage is set to 2.3 volts, that is a levelbetween the threshold level V_(T2) and V_(T1). Also it must be set at alevel which is less than the minimum current to be sensed when the wordline voltage is 2.8 volts and the threshold is V_(T2) or V_(T1).However, it is used for sensing a transition between current conductingand virtually no current conducting. Thus, the margin for error in thereference current I₀ is relatively large.

The margin for error in the reference current I₁ is similarly great,because it is sensing between the current of a cell having the lowestthreshold voltage V_(T0) and the current from a cell having thethreshold voltage V_(T1). The margin of safety in the current I₁ can beincreased by making the threshold voltage V_(T0) much less than thethreshold voltage V_(T1). In the preferred embodiment, the thresholdvoltage V_(T0) is approximately 0.8 volts, while the threshold voltageV_(T1) is approximately 2.0 volts. This provides significant margin forsafety in the reference current I₁. FIG. 5(A) illustrates a senseamplifier in a preferred embodiment in the present invention. In FIG.5(A), the cell current I_(CELL) is provided on the data line 510. Thedata line 510 is coupled to the input of inverter 501 and to the sourceof transistors 502 and 503 respectively. The gates of transistors 502and 503 are coupled to the output of the inverter 501. Thus, when thedata line 500 is in a condition for sensing, the inverter 501 will drivethe gates of transistors 502 and 503 into the on condition allowingcurrent from the cell to be provided to the sense amplifier at nodes 520and 525 respectively. First reference current I₀ is supplied fromcurrent source 504, a second reference current I₁ is provided fromcurrent source 505. The first reference current I₀ is coupled to node525. Node 525 is connected to the input of inverter 550. The output ofthe inverter 550 provides the output of the current comparator for thereference current I₀. Node 520 is coupled to the input of inverter 530.The output of inverter 530 provides the output of the current comparatorfor the node 520 and the reference current I₁. The output of inverter530 is connected to the input of NAND gate 540. The second input to NANDgate 540 is the address bit XM. The output of the NAND gate 540 isconnected to a switch, SW1, and through inverter 560 to a second switch,SW2. The switch SW1 couples the output of the inverter 550 to the outputS_(OUT) of the sense amplifier directly. If switch SW1 is open andswitch SW2 is closed, then the output of the inverter 550 is suppliedthrough inverter 570 as the output S_(OUT) providing the output of thecurrent comparison at node 525 inverted as the output S_(OUT). Asillustrated in the Figure, when the address bit XM is zero, then theoutput of the NAND gate is high independent of the output of theinverter 130. This in effect maintains switch SW1 closed during thesensing of the low bit and switch SW2 open. Thus, the low bit isindicated by the output of the comparator of the cell current with thereference current I₀.

When XM=1, two conditions may occur as illustrated in FIGS. 5(B) and5(C) respectively. In the first condition the output of the comparatorfor node 520 is zero. In this case, the switch SW1 is open and theswitch SW2 is closed causing inverter 570 to be inserted into the pathbetween the output of the comparator for node 525 and the outputS_(OUT). When the output of the comparator for the node 520 is one asillustrated in FIG. 5(C), then the logic causes the switch SW1 to beclosed and the switch SW2 to be open. Thus the output of the comparatorfor the node 525 is coupled directly as the output S_(OUT) of the senseamplifier.

When XM=0, two conditions may occur as illustrated in FIGS. 5(D) and5(E) respectively. In the first condition the output of the comparatorfor node 520 is zero. In this the logic causes the switch SW1 to beclosed and the switch SW2 to be open, thus the output of the comparatorfor the node 525 is coupled directly as the output S_(OUT) of the senseamplifier. If the output of the comparator for the node 520 is one asillustrated in FIG. 5(E), the logic causes the switch SW1 to be closedand the switch SW2 to be open, thus the output of the comparator for thenode 525 is again coupled directly as the output S_(OUT) of the senseamplifier.

According to the sequence illustrated in FIGS. 6(A)-6(F) a chip enablesignal CEB transitions to the low state at time 600. The address bit XMand the other address bits are valid during an interval between the time600 and the time 601. If the address bit XM is high during that intervalas indicated in the figure, the word line voltage is raised to a levelof about 2.8 volts, corresponding to a level above the threshold voltageV_(T2) and below V_(T3), and reaches that level at time 602. Also, theground line connected to the selected cell is raised to a voltage levelof about 0.5 volts and reaches that level at a time of about 603. Dataout is sensed during the interval beginning at time 604 and ending attime 605 for the low bit when address bit XM is high. During thisinterval, the gate-to-source voltage is defined by the differencebetween the word line voltage and the ground line voltage is at about2.3 volts or a level between the threshold voltages Vt₁ and Vt₂. Duringa next cycle, or if the address bit XM is low independent of othercycles, the word line voltage is maintained again at the fixed level of2.8 volts which is a level above the threshold voltage Vt₂. However, forthe high bit, the ground line voltage is set during the interval 606 toa level of about 0 volts. Thus, during sensing of the high bit in theinterval from time 608 to time 609, the gate-to-source voltage is fixedat a level of about 2.8 volts, above the threshold voltage Vt₂. Thesense amplifier described above is utilized for sensing the low bit andthe high bit according to this scheme. One advantage of the schemeillustrated in FIGS. 6(A)-6(F) is that the word line driver can besimplified to provide a single level during sensing. It comes at thecost however of a source voltage supply, such as the supply 141 of FIG.1, which must be able to switch between 0.5 volts and ground, or othersimilar levels in response to the address bit XM.

FIGS. 7(A)-7(F) illustrate yet another alternative embodiment. Accordingto the embodiment of FIGS. 7(A)-7(F), a single current comparator can beutilized in the sense amp. This is accomplished by the followingsequence. As with other embodiments, when the chip enable signal fallslow at time 700, the address bit XM and other address bits become validduring the interval between time 700 and time 701. If XM is high, a lowbit is sensed and a word line voltage is raised to a level of about 2.3volts in this example at time 702. The ground line voltage is maintainedat ground for this example. In this way, the gate-to-source voltage isfixed at a level between the threshold voltage Vt₁ and Vt₂ sensing thelow bit during the interval between times 703 and 704. This isaccomplished by determining whether the current at the sense amplifierfrom the cell falls above or below the reference current I_(REF). Duringthe sensing of the high bit, as indicated during the interval betweentime 701 and time 705, the address bit XM is low. This time, the wordline voltage is raised to a value of about 2.8 volts at time 706 in thisexample. The high bit is sensed in a first step or phase and a secondstep or phase. During the first phase 707, the ground line voltageremains near 0 volts. During the second phase 708 the ground linevoltage is raised to a level of about 1.0 volts. The sense amplifierdetects whether the current from the cell exceeds the first referencecurrent during the first phase, indicating that the threshold voltage ofthe cell is below the threshold Vt₂, and during the second phase againindicating whether the threshold voltage of the cell is below thedifference between the voltage on the word line and the voltage on theground line which is established at a level below Vt₁ and above Vt₀.Thus, this technique allows for sensing a low bit in a multi-bit memorycell by determining whether the threshold voltage of the cell fallswithin a high group having a threshold above 2.3 volts, and a low grouphaving a threshold below 2.3 volts for this example. During sensing ofthe high bit, this technique allows using a single reference currentsource for determining whether the threshold voltage of the memory cellfalls within the outer group having a threshold voltage above 2.8 voltsor below 1.8 volts, or has a threshold voltage between 2.8 volts and 1.8volts. The high bit and the low bit can be sensed independent of oneanother using independent gate-to-source voltage cycles.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

What is claimed is:
 1. A method for sensing a particular bit in aplurality of bits in a multibit cell having a threshold voltage within arange of one of a plurality of predetermined threshold voltages whichincludes a highest, a next to highest and a lowest predeterminedthreshold voltage, comprising: applying a gate-to-source voltage to themultibit cell between the highest and the next to highest predeterminedthreshold voltages to produce a cell current; and determining a firstvalue for the bit if the cell current is less than a first referencecurrent indicating that the threshold voltage is the highestpredetermined threshold voltage, or if the cell current is greater thanthe first reference current and greater than a second reference currentindicating that the threshold voltage is the lowest predeterminedthreshold voltage, and a second value for the bit if the cell current isgreater than the first reference current and less than the secondreference current indicating that the threshold voltage is one of theother predetermined threshold voltages in the plurality of predeterminedthreshold voltages.
 2. The sensing method of claim 1, wherein thedetermination of the first and second values indicate two bits of theplurality of bits stored in the multibit memory cell.
 3. The sensingmethod of claim 1, further comprising the steps of: receiving at a firstcomparator the first reference current and the cell current; receivingat a second comparator the second reference current and the cellcurrent; and providing an output representing the two bit data based onlogic coupled to the first and second comparators, and operating duringa first time interval for providing the output of the first comparatoras the first output, and operating during a second time interval forproviding the output of the first comparator as the second output if theoutput of the second comparator has a first value, and providing theoutput of the first comparator inverted as the second output if theoutput of the second comparator has a second value.
 4. The sensingmethod of claim 1, the method further comprising the steps of: receivingan nth reference current at an nth comparator and the cell current;receiving a (n+1)th reference current at a (n+1)th comparator and thecell current; and providing an output based on logic coupled to the nthand (n+1)th comparators, operating during an nth time interval forproviding an output of the nth comparator as the nth output, andoperating during the (n+1)th time interval for providing the output ofthe nth comparator as the (n+1)th output if the output of the (n+1)thcomparator has a first value, and providing the output of the nthcomparator inverted as the (n+1)th output if the output of the (n+1)thcomparator has a second value, wherein n is an integer equal to 2, 3, 4,. . .
 5. The sensing method according to claim 1, wherein determinationof the first value for the bit is performed during a time intervaloccurring before a read cycle for determining the second value for thebit.
 6. The sensing method according to claim 1, wherein determining ofthe second value for the bit is determined during a time intervaloccurring before a read cycle for determining the first value for thebit.
 7. A method for sensing data in a multibit cell having a gate, asource and a drain, and having a threshold gate-to-source voltage withina range of one of a plurality of predetermined threshold voltages,comprising: selecting a bit in the multibit cell in response to a selectsignal; applying a gate-to-source voltage across the gate and source ofthe multibit cell, in response to the selected bit; and detecting duringthe step of applying the gate-to-source voltage a state of the selectedbit.
 8. The method of claim 7, wherein the step of applying agate-to-source voltage, includes applying a first predetermined gatevoltage to the gate of the multibit cell in response to selection of afirst bit stored in the multibit cell, and applying a secondpredetermined gate voltage to the gate of the multibit cell in responseto selection of a second bit in the multibit cell.
 9. The method ofclaim 8, wherein the step of applying a gate-to-source voltage, includesapplying a predetermined source voltage to the source of the multibitcell in response to selection of either the first bit or the second bit.10. The method of claim 7, wherein the step of applying a gate-to-sourcevoltage, includes applying a first predetermined source voltage to thesource of the multibit cell in response to selection of a first bitstored in the multibit cell, and applying a second predetermined sourcevoltage to the source of the multibit cell in response to selection of asecond bit in the multibit cell.
 11. The method of claim 10, wherein thestep of applying a gate-to-source voltage, includes applying apredetermined gate voltage to the gate of the multibit cell in responseto selection of either the first bit or the second bit.
 12. A method forsensing data in a multibit cell having a gate, a source and a drain, andhaving a threshold gate-to-source voltage within a range of one of aplurality of predetermined threshold voltages, comprising: selecting afirst bit or a second bit in the multibit cell in response to a selectsignal; detecting a state of the selected bit if the first bit isselected by determining whether the threshold voltage of the memory cellfalls within a high threshold group including threshold voltages higherthan a first level of the gate to source voltage indicating one binarystate of the selected bit, or within a low threshold group includingthreshold voltages lower than the first level indicating another binarystate of the selected bit, and if the second bit is selected bydetermining whether the threshold voltage of the memory cell fallswithin an inner threshold group including threshold voltages between asecond level and third level indicating one binary state of the selectedbit and an outer threshold group including threshold voltages greaterthan the second level and less than the third level indicating anotherbinary state of the selected bit.
 13. The method of claim 12, whereinthe step of detecting includes: applying a gate-to-source voltage acrossthe gate and source of the multibit cell, in response to the selectedbit, the gate-to-source voltage fixed at or near the first level if thefirst bit is selected, and fixed at or near the second level if thesecond bit is selected.
 14. The method of claim 13, wherein the step ofdetecting includes comparing current from the cell to a first referencecurrent if the first bit is selected, and includes comparing currentfrom the cell to the first reference current and to a second referencecurrent if the second bit is selected.
 15. The method of claim 13,wherein the step of applying a gate-to-source voltage, includes applyinga first predetermined gate voltage to the gate of the multibit cell inresponse to selection of the first bit stored in the multibit cell, andapplying a second predetermined gate voltage to the gate of the multibitcell in response to selection of the second bit in the multibit cell.16. The method of claim 15, wherein the step of applying agate-to-source voltage, includes applying a predetermined source voltageto the source of the multibit cell in response to selection of eitherthe first bit or the second bit.
 17. The method of claim 13, wherein thestep of applying a gate-to-source voltage, includes applying a firstpredetermined source voltage to the source of the multibit cell inresponse to selection of the first bit stored in the multibit cell, andapplying a second predetermined source voltage to the source of themultibit cell in response to selection of the second bit in the multibitcell.
 18. The method of claim 17, wherein the step of applying agate-to-source voltage, includes applying a predetermined gate voltageto the gate of the multibit cell in response to selection of either thefirst bit or the second bit.
 19. The method of claim 12, wherein thestep of detecting includes: applying a gate-to-source voltage across thegate and source of the multibit cell, in response to the selected bit,the gate-to-source voltage fixed at or near the first level if the firstbit is selected, and applied in a sequence if the second bit is selectedincluding a first step fixed at or near the second level, and a secondstep at or near the third level.
 20. The method of claim 19, wherein thestep of detecting includes comparing cell current to a first referencecurrent if the first bit is selected to determine a binary state of thefirst bit, and comparing the cell current to the first reference currentif the second bit is selected during the first and second steps.
 21. Themethod of claim 19, wherein the step of applying a gate-to-sourcevoltage, includes applying a first predetermined gate voltage to thegate of the multibit cell in response to selection of the first bitstored in the multibit cell, and applying a second predetermined gatevoltage to the gate of the multibit cell during the first and secondsteps in response to selection of the second bit in the multibit cell.22. The method of claim 21, wherein the step of applying agate-to-source voltage, includes in response to selection of the firstbit applying a first predetermined source voltage to the source of themultibit cell, and in response to selection of the second bit applying asecond predetermined source voltage to the source of the multibit cellduring the first step and a third predetermined source voltage to thesource of the multibit cell during the second step.
 23. The method ofclaim 22, wherein the first and second predetermined source voltages aresubstantially equal.